A reported mobile phone surveillance program at the U.S. Department of Justice raises serious privacy questions, a U.S. senator said Monday.

The DOJ program, which reportedly uses cell-tower mimicking equipment on airplanes to target the mobile phone locations of criminals, raises questions about how many “innocent” people’s mobile phone data is also swept up in the operation, said Senator Ed Markey, a Massachusetts Democrat. The DOJ has not confirmed the existence of the surveillance program, reported in a Wall Street Journal article on Thursday.

“Americans are rightfully disturbed by just how pervasive collection of mobile phone information is, even of innocent individuals,” Markey said in a statement. “While this data can be an important tool for law enforcement to identify and capture criminals and terrorists, we must ensure the privacy rights of Americans are protected.”

Markey on Monday disclosed a letter he sent to Attorney General Eric Holder asking for details about the program.

“We need to know what information is being collected, what authority is being used to collect it, and if and how this information is retained and stored,” said Markey, a long-time advocate of personal privacy.

In the letter, Markey asks Holder how long the surveillance program has operated and if the DOJ has any other mobile phone surveillance programs that use airplanes, helicopters or drones. He asks how many times the DOJ has conducted mobile phone surveillance operations over U.S. cities.

The senator also asks if the DOJ has a court order that allows the surveillance and whether the agency has informed judges about the number of “innocent people whose information may be swept up.”

He also asks what kind of information the program collects and whether there are any limitations on what kinds of investigations the surveillance can be used for.